The peaks of random access signals used for random access in a 3GPP-LTE system exist as illustrated in FIG. 1, if the random access signal is accurately synchronized with a base-station reference signal. FIG. 1 shows a preamble sequence, and the random access signal is accurately synchronized with the base-station reference signal when the peak of the random access signal exists in each of index values 0, CS, 2×CS, 3×CS and V_MAX×CS. Preamble sections P1, P2, P3 and P0 correspond to preamble index values IDX_P1, IDX_P2, IDX_P3 and IDX_P0 respectively.
However, there must be a delay equal to a time taken for the random access signal to travel from the terminal to the base station, because the random access signal is not synchronized with the base station but with a downlink signal only, because it is a very first signal that a terminal sends to access a base station. Such a delay allows the position of a peak to be shifted to the left. In FIG. 1, a Cyclic Shift (CS) value is designed to be larger than the shift value of a peak value where the delay of the random access signal is the maximum, in consideration of a radius of a cell. Accordingly, when there exists a peak in the range of the index values 1 to CS, the base station determines that the peak existing in the index value CS has been shifted by the delay and determines it as a random access signal corresponding to the peak existing in the index value CS. That is, the base station determines it as the random access signal of a first preamble section P1. Meanwhile, the respective preamble sections P1, P2, P3 and P0 have different preamble index values IDX_P1, IDX_P2, IDX_P3 and IDX_P0. The base station calculates the preamble index values IDX_P1, IDX_P2, IDX_P3 and IDX_P0, and determines which of the preamble sections P1, P2, P3 and P0 a received random access signal is in. As another example, when a peak occurs between the index values CS+1 and 2×CS, the base station determines that the peak in the index value 2×CS has been shifted. That is, the base station determines it as the random access signal of a second preamble section P2. A random access signal, in which a peak exists in the range of the index values CS+1 to 2×CS, has a preamble index value different from a preamble index value of a random access signal where a peak exists in the range of the index values 1 to CS.
As illustrated in FIG. 1, a random access signal fades and spreads sideways due to noise while passing through a wireless channel. According to the extent of spread, a peak can be classified into a narrow peak and a wide peak. FIG. 2 illustrates the shapes of the narrow peak and the wide peak. A narrow-peak signal has only one peak index value exceeding a threshold value, but, on the other hand, a wide-peak signal has one or more peak index values exceeding the threshold value.
FIG. 3 illustrates a received random access signal which is a narrow-peak signal and shifted by a delay. Herein, the peak index value exceeding the threshold value exists in an index value 2×CS−1. In this case, it is determined that a peak existing in the index value 2×CS is shifted, and thus the random access signal is detected.
As shown in FIG. 4, in a case where a delay exists in the wide-peak signal, an index value exceeding the threshold value widely exists around a reference-peak index value in a random access signal. In this case, when a peak is detected, a peak existing in index values 2×CS+1 to 2×CS is determined as a signal, which is a random access signal existing in the index value 2×CS, received with or without delay, and a random access signal existing in the index value 2×CS+1 is determined as a signal that is generated by the delay of a random access signal existing in the index value 3×CS. At this point, a false alarm occurs due to the spread of a peak value.
There is an attempt to increase the threshold value as one of methods for decreasing a false alarm. However, the method of increasing the threshold value makes it difficult to detect the signal when the amplitude of the random access signal is small, and thus decreases the probability of detection.